It has been found that, when the brakes of a transit vehicle or railway train are applied, a braking force must be properly controlled in order to safely and efficiently slow down and/or stop the vehicle or train at a station or the like. For a given running surface condition, the force between the wheel tread and the running surface increases which intensifies the slip value toward a critical wheel slip value. When the value of the wheel slip increases beyond the critical slip value, the force between the wheel tread and running surface decreases. It will be appreciated that stable and effective braking occurs when the slip value is equal to or less than the critical slip value. Thus, when the slip value becomes greater than the critical slip value, the braking becomes unstable and results in a sudden wheel lockup which can increase the actual stopping distance. Accordingly, in the braking operation, it is advantageous to detect an incipient wheel lockup by continuously monitoring the wheel slip value between the wheel tread and the running surface in order to achieve the maximum possible braking effort.
A prior art wheel-slip control system is shown and disclosed in our U.S. Pat. No. 4,491,920, issued on Jan. 1, 1985, entitled "Rate Polarity Shift Wheel-Slip Control System," which is assigned to the assignee of this invention and which is incorporated by reference to the present application. Briefly, the wheel-slip control system is for a multiple-truck vehicle including a speed sensor for generating signals representative of the velocity of each of the wheel axle units. A differentiator is connected to each of the speed sensors for differentiating the velocity signals to obtain rate signals. A rate-determining circuit determines the most negative-going rate signal of each of the wheel axle unit of each truck. A plurality of deceleration threshold and rate direction detectors and data processing logic initiate a brake force reduction action on the truck experiencing a wheel slip, and a positive logic "OR" gate senses a polarity shift in the most negative rate signal to cause the data processing logic to reapply a braking action.